Organic peroxide-curable and organic peroxide-cured synthetic polyester rubbers



Patented Sept. 7, 1948 ORGANIC PEROXIDE -CURABLE AND OR- GANICPEROXIDE-CURED SYNTHETIC POLYESTER RUBBERS Calvin S. Fuller, Chatham, N.J., asslgnor to Bell Telephone Laboratories,

Incorporated,

New York, N. Y., a corporation of New York No Drawing. Application April30, 1943,

Serial No. 485,202

3 Claims. (Cl. 260-75) This invention relates to cured synthetic rubbersand to substances capable of being cured to synthetic rubbers. Thisapplication is in part a continuation of the application of 0.8. Fuller,Serial No. 401,956, filed July 11, 1941, now abandoned.

The cured synthetic rubbers of the present invention are prepared bycross-linking polyesters of high molecular weight which possessinsufilcient crystallinity to render them brittle and particularly thosewhich are essentially noncrystalline, plastic gums at room temperatures.This curing is accomplished by intimately mixing the polyester with asubstance which is capable of generating free radicals having acrosslinking activity and heating the mixture to decompose the freeradical generating substance and cause cross-linking. Benzoyl peroxideis a typical and outstanding example of such a free radical generatingsubstance.

These polyesters may be strictly linear polyesters of high molecularweight containing no non-benzenoid carbon-to-carbon unsaturat ion, whichare prepared by the superesteriflcation of a glycol with a dicarboxylicacid of a hydroxy acid with itself, in a manner similar for instance, tothat described for crystalline polyesters in U. S. Patents 2,071,250 and2,249,950. Polyesters, which are essentially similar but which containlimited amounts of olefinic or non-benzenoid unsaturation, may also becured to form valuable synthetic rubbers. These partially unsaturatedpolyesters may be prepared in the same manner as the fully saturatedpolyesters, except that one or more of the ingredients of the reactionmixture from which they are prepared contains a properly limited amountof unsaturated carbonto-carbon bonds.

The polyester gums, the preparation of which will be described in moredetail below, are extremely viscous liquids, which at room temperaturehave a consistency somewhat similar to that of milled crepe rubber, orelse they are somewhat flexible, rubbery solids of slight crystallinitywhich melt readily at temperatures up to about 20 C. above roomtemperature to form viscous liquids of a consistency similar to thenormally liquid polyesters. These latter substances possess suflicientflexibility. being largely amorphous, to be milled directly on coldrolls, where they are almost instantly reduced to a viscous liquid stateby the temperature rise induced by milling.

These polyesters, both liquid and solid, are prepared for vulcanizationby milling the polyesters, preferably on cool rolls, with a curingagent, such finely divided powder. Milling is facilitated by.

as benzoyl peroxide, preferably in the form of a a temperaturesuflicient to cross-link the polyester. Since curing is practicallyinstantaneous, the curing operation need not be continued substantiallylonger than the time necessary for the interior of the molded article toreach the curing 1!! temperature.

The resulting cured, pigmented rubber, when m prepared from suitablepolyesters and reinforcing pigments properly proportioned and whenproperly cured, may achieve tensile strengths as high as 3,000 poundsper square inch, or even higher,

at elongations in the vicinity of 650 per cent. The cured substanceshave an excellent resistance to gasoline and hydrocarbon oils, being farsuperior to natural rubber and superior to other synthetics in thisrespect. They are resistant polyesters.

to many corrosive substances, such as sulphur, chlorine and fluorine,which rapidly deteriorate other rubber-like materials. They also have aresistance to dry heat, particularly in the absence 01' oxygen, which isfar superior to natural rubber and other synthetic elastomers. By, aproper choice of polyesters, cured substances may be obtained havingbrittle points at low temperatures approaching those of natural rubber.Because of the extreme plasticity of the uncured polyesters at elevatedtemperatures, excellent intricate moldings can be obtained. In theirabrasion resistance and electrical properties, the rubbers of thepresent invention are adequate for most uses. Their susceptibilityto-hydrolysis precludes their use at high temperatures in the presenceof moisture, but in most cases is not suflficient to interfere withtheir use under normal conditions.

Reversible elasticity and good tensile strength in the cured rubbers ofthe present invention are dependent upon certain properties in theuncured Since failure of the rubber under tension occurs ordinarily fromthe overcoming of forces holding different molecules together. ratherthan from. the internal splitting of the molecules, any factors tendingto increase the forcesbetween molecules will increase the tensilestrength. One of the most important factors in determining the tensilestrength of the cured gum itself is the degree of linear growth of themolec es of the polyester. With the strictly linear polyesters preparedfrom glycols and dicarboxylic acids containing no non-benzenoidunsaturation or from monohydroxy monocarboxylic acids containing nonon-benzenoid unsaturation, the degree oi linear growth is measureddirectly by the molecular weight of the polyester, since theoreticallyeach molecule is made up of a single long chain.-

There is a relatively sharp increase in the tensile strength 01 thecured polyesters when the molecular weights of the polyester gums fromwhich they are prepared achieve values in the vicinity of 8,000 to10,000 as estimated by the Btaudinger viscosity method. The usefulrubbers of the present invention are prepared from polyesters havingmolecular weights of at least this minimum and usually from polyestershaving higher molecular weights. Linear polyesters of such molecularweights ordinarily possess intrinsic viscosities in chloroform of atleast 0.4.

The degree of linear growth for saturated linear polyesters may also beexprewed in terms of the average number of atoms in the linear chains ofthe polyester molecules. To possess the requisite tensile strength thepolyesters of the present invention'shoul-d contain an average of atleast 500 or 600 atoms in their linear chains.

Since the polyesters are prepared from the esterification oi onlybiiunctional ingredients, the degree of linear growth can also beexpressed in terms of the degree of esterification possessed by thepolyester. The most suitable products for the present invention areproduced when the polyesters possess a degree of esterifi-cation inexcess of 98 per cent. Polyesters having this degree of esterificationwill contain at least 98 ester groups for each 100 total ester, hydroxyland carboxyl groups in the polyester. j

Although linear growth has an important eflect in increasing theadhesive force'between molecules and thus increasing tensile strength,other details of molecular structure also play a part in this result.One of the most important of these factors is the degree of order in themolecular chains of the polyester. Thus polyesters in which the polargroups are regularly spaced ordinarily have a higher tensile strengththan those which havea more random spacing. Thus polyesters producedfrom a single hydroxy acid have regularly spaced polar groups and have ahigher tensile strength than those produced from a single glycol and asingle dicarboxylic acid of diflerent chain length. If more than oneglycol or more than one dicarboxylic acid is employed for producing thepolyester, the degree of order becomes lower and the tensile strengthbecomes less. However, since the hydroxy acid-s are not availablecommercially, the polyesters produced from. glycols and dicarboxylicacids are considerably more important technically. It is desirable,however, from the tandpoint of tensile ..strength, to limit as far aspossible the total number of glycols and dicarboxylic acids going into asingle polyester.

It has also been iound that polyesters containing no non-benzenoidunsaturation give cured products having higher tensile strengths thanthose containing such unsaturation. This eilect oi' unsaturation isaggravated by the fact that unsaturation is most easily introduced intothe polyester by the substitution of an unsaturated dicarboxylic acidfor a portion of the saltura ted acid, thus causing a lower degree oforder 20 since these substances,

a in the molecular chain and a lower tensile stren th.

Although the fully saturated polyesters are therefore more desirablefrom the standpoint of 5 tensile strength, a considerably greater amount0! curing agent is required for their vulcanization than for thevulcanization oi polyesters containing unsaturation. The necessity tor agreater amount of curing agent increases the cost of the cured productand causes larger amounts of undesirable by-products to be left in thecured product. The presence of these by-products ordinarily shortens thelife of the rubber. Therei-ore, the partially unsaturated polyesterswill oflten be found more desirable technically.

itaconio, mesacon-ic, muconic and dihydro- :muconic acids.

In order to (produce the necessary high degree of esterlfication'orcondensation indicated above as required for the proper linear growth,the reactants from which the polyesters are produced must :be subjectedto a prolonged heating operation under conditions such as to remove thereaction by-products continuously and criestively. The reactionby-products are most effectively removed by bubbling an inert gas suchas dry, oxygen-tree hydrogen through the reaction mix'ture until theesteriflcation or condensation has proceeded to the desired degree ofcompletion, with or without the application of 40 reduced pressure. Thereaction by-products may also be removed by other means, such as by theuse of a, molecular still or by stirring the reaction mixturecontinuously into a foam under reduced pressure. Unless some such methodis employed,

the high degree of condensation necessary for the production ofrubber-like polymers oi good tensile strength can not be obtained withina reasonable time.

When olefinic 'bonds are present in the reaction mixture, double bondpolymerization occurs, si-

multaneously with the condensation reaction responsible for lineargrowth. If too great a number of such double bonds is present, thepolymerization reaction will proceed at such a rate simultaneously withthe esterification reaction that the viscosity of the mixture willincrease excessively to a point where the product is no longer capableof permanent distortion and is no longer liquid. Ii. this excessivecross-linking re- 0 action occurs before the mixture has been esterifiedto the required degree, it will be impossible to secure a plasticproduct capable of being compounded and vulcanized to a synthetic rubberof the desired high tensile strength. Moreover,

with too high a degree of unsaturation in the Polyester, the cured lowelon-gations.

In general, it may be stated that the amount of any substance in thereaction mixture containproducts have undesirably ing oleflnic bondsshould be so limited that the number of bonds is less than about 5 per400 atoms in the linear chain 01' the theoretical resulting polyesterand preferably less than about 2 such bonds per 400 atoms in the linearchain.

This limitation is based upon the average theoretical molecule produced,assuming that all the unsaturated bonds in the initial reactants remainas unsaturated bonds in the polyester which is produced and that nocross-linking occurs.

When a dicarboxylic acid containing conjugated unsaturated bonds, suchas maleic acid, is used for forming the unsaturated polyesters of thepresent invention, this limitation of the unsaturation in the resultingpolyester may be accomplished by diluting the unsaturated acid with adicarboxylic acid containing no unsaturated bonds, such as succinic acidor sebacic acid, and esterifying the resulting mixture with glycol. Ingeneral, in such a mixture the maleic acid should not exceed per cent byweight of the mixture of acids and is preferably in the vicinity ofabout 5 per cent by weight or less. Where it is desired to takeadvantage of the property of curing with auaaas a substantially smalleramount of curing agent, 7

the maleic acid should constitute at least 1 per cent of the totaldicarboxylic acids" employed.

In view of this tendency of esterification mixtures containingunsaturated bonds to become set due to prolonged heating before thedesired degree of linear growth is obtained, it is desirable-to use anesterification technique which will consistently require a minimum oftime. When the polyester is formed from glycols and dicarboxylic acids,this can be accomplished by employing a large excess of glycol in theinitial reaction mixture, preferably between about 5 per cent excess andabout 50 per cent excess, and carrying out the initial esterificationreaction in a vessel equipped with a reflux condenser which ismaintained at a temperature sufllcient to permit the escape of watervapor while returning the greater part of the vaporized glycol to thereaction mixture. conducted at a temperature between about 180 C. andabout 220 C. and preferably in the vicinity of about 200 C. The largeexcess of glycol which is retained in the reaction mixture pushes theesterification reaction rapidly to completion leading to the formationof a relatively low molecular weight polyester, substantially all themolecules of which have hydroxyl groups at both ends.

linear growth is much more difilcult for these unsaturated polyestersthan for the strictly linear molecules.

It appears that the unsaturated polyester molecule is made up ofessentially linear molecule chains which are produce'd by esteriilcationand that these linear chains are linked at various points. If thereaction mixture is allowed to unclergo a degree of esterification, orof condensation by ester interchange, equal to that of'a saturatedreaction mixture, the resulting polyester will possess essentially thesame degree of linear growth. Therefore the most convenient means fordefining the minimum degree of linearity. for partially unsaturatedpolyester gums, is'to require that the average polyester-moleculecontain at least98 ester groups per 100 total ester, hydroxylandcardboxyl groups or that the polyester have a degree of esteriflcationof at least 98 per cent. 1

when polyesters are produced from reaction mixtures containing more than10 mol per cent of an unsaturated dlcarboxylic acid (or more than about5 unsaturated bonds per 400 atoms in the linear chain) and up to aboutmol per .cent unsaturated acid, a degree of esterificationmay beachieved without gelation which, whenthe polyester is cured, will yieldmoderate tensile strengths. However, due to the large amount ofunsaturation present, the curing of such polyesters by the processdescribed above is extremely sensitive and critical, making it verydifllcult, and

' usually impossible, to prevent overcuring to an The esterificationreaction is extent which produces substances of very low elongationsmore [closely resembling, in their physical properties, linoleums thanrubbers.

Polyesters of these high molecular weights will produce cured rubbershaving good reversible elasticity only if the polyesters are capable offlow at room temperature or at temperatures not substantially higherthan about 20 C. above Further linear growth can then proceed only moreparticularly described and claimed in the copending application of J. B.Howard, Serial No. 492,155, filed June 24, 1943, now United StatesPatent 2,410,073 issued October 29, 1946.

As indicated above, when polyesters are formed from reactants at leastone of which contains non-benzenoid unsaturation, a partialcross-linking reaction takes place simultaneously with theesteriflcation reaction. If this cross-linking is prevented frombecoming excessive through limitation of the amount of unsaturationinitially present and through control of the reaction conditions, lineargrowths comparable to those obtained with fully saturated linearpolyesters may be produced. Because of the complexity of themolecularstructure, definition of the degree of room temperature. Sincepolyesters possessing a high degree of crystallinity are essentiallyrigid, good rubbers can be obtained only from polyesters which areessentially non-crystalline at room temperatures. Polyesters whichpossess a small amount of crystallinity, sufllcient substantially todestroy their property of flow under moderate stress, are neverthelesssuitable for the purposes of the present invention, provided theircrystallinity isnot suflicient to render them hard and brittle andprovided their crystalline melting point is not g'reaterthan about 20,0. above room temperature. The crystallinity of such polyesters isreduced by the process of vulcanization so that in many cases they maybehave, at room temperatures, not substantially different from the curedpolyesters which were originally viscous liquids. Even when thecrystallinity remaining after vulcanization is sufllcient to render theproducts boardy at room temperature or below, these substances are notbrittle since the heat generated by distortion under stress issuflicient to reduce or destroy the crystallinity rapidly and thusproduce true rubberlike behavior very shortly after the application ofthe stress. These partially crystalline polyesters possess an advantageover the viscous liquid polyesters in that, prior to curing, they may bestored or shipped without adhering to their containers.

In producing such non-crystalline polyeazters or polyesters of limitedcrystallinity, advantage is taken of the fact that certain ingredientslead to polyesters which are incapable of crystallization or which havecrystalline melting points below room temperatures or which crystallizeso slowly that for practical purposes they may be considered permanentlynon-crystalline.

Polyesters derived by the esterification of polymethylene glycols withpolymethylene dicarboxylic acids or by the esterification ofpolymethylene monohydroxy monocarboxylic acids are, with the exceptionof those produced from trimethylene glycol and glutaric acid, the mosthighly crystalline polyesters which have been produced. As the molecularstructure departs from this straight chain polymethylene arrangement, asfor; instance by the introduction of side chain substituents,hetero-atoms or unsaturated carbon-to-carbon bonds, the polyestersbecome less crystalline. The presence of aromatic rings also in generalreduces the crystallinity.

Therefore, polyesters prepared by reacting glycols with dicarboxylicacids, where either one of the constituents has frequently occurring orlarge side chains, or contains large amounts of non-benzenoidunsaturation or aromatic rings or hetero-atoms in the linear chain, areusually noncrystalline. However, if the other member of the reactionmixture is a polymethylene glycol or a polymethylene dicarboxylic acidthe crystallizing tendencies of the polyester increase as the length ofthe .polymethylene chain increases. Thus dihydromuconic acid forms anon-crystalline polyester with ethylene glycol but a crystallinepolyester with decamethylene glycol. Diethylene glycol forms anon-crystalline polyester with succinic acid but a crystalline polyesterwith sebacic acid.

Among the alkyl substituted polymethylene glycols, the most available isisopropylene glycol of methylethylene glycol. This glycol formsnoncrystal-line polyesters with polymethylene dicarboxylic acids betweensuccinic acid and sebacic acid. Polyispropylene succinate does notbecome excessively crystalline when as much as 50 or 60 per cent of theisopropylene glycol is replaced by ethylene glycol. With isopropylenesebacate. however, no more than about 30 per cent of ethylene glycol canbe substituted for the isopropylene glycol without inducing excessivecrystallization.

Although dicarboxylic acids containing conjugated unsaturation, such asmaleic or fumaric acid, form non-crystalline polyesters with the commonpolymethylene glycols, they are used in such small concentrations in thepolyesters of the present invention that their efiect upon thecrystallinity is not great. v

The most readily available of the non-crystalline polyester formingreactants containing hetero-atoms are.diethylene glycol anddiisopropylene gylcol. Diglycolic acid is also of some interest as ahetero-atom containing compound.

The most available of the aromatic ring containing reactants is phthalicacid.

Trimethylene glycol and g lutaric acid, both of which contain threemethylene groups between their functional end groups, form polyesters,with the shorter chain polymethylene glycol and polymethylenedicarboxylic acids, which crystallize exceedingly slowly and aretherefore useful for forming certain of the cured synthetic rubbers ofthe present invention.

Another factor influencing crystallinity, aside from themolecularstructure of the individual constituents, is the degree of order in thepolyester molecules. The most ordered molecules having the most regularpolar group spacing, all other factors being equivalent, are the mostcrystalline. Thus, the greater the number of glycols and the greater thenumber of dicarboxylic acids or the greater the number of hydroxy acidsused in preparing the polyester. the less will be the tendency tocrystallize. In a polyester prepared from etheylene glycol and equimolaramounts of sebacic and succinic acids. or similar polyesters in whichmaleic acid is substituted for portions of the succinic acid, thedisorder imparts suificient non-crystallinity to permit the polyester tobe used for the purposes of the present invention.

Even though a polyester is non-crystalline, the utility of the curedproduct may be limited by its brittle point. The temperature below whichthe cured rubber becomes brittle is closely associated with the.temperature at which the viscosity of the polyester becomes so greatthat the material no longer flows readily under pressure but becomesinstead a brittle, glassy substance. The degree to which a rubber mustbe capable of being cooled before becoming brittle is dependent upon theuse to which it is to be put. However,

substances having a brittle temperature at or above room temperature areobviously of limited utility since their rubber-like properties can betaken advantage of only at elevated temperatures.

The non-crystalline polyesters formed predominantly from polymethyleneglycols and polymethylene dicarboxylic acids, and from such glycols anddicarboxylic acids having substituted alkyl side chains, form curedproducts which invariably have brittle points well below roomtemperature. However, those polyesters made from reaction mixtures inwhich the dicarboxylic acid is almost all phthalic acid are usuallybrittle glasses at room temperature and form cured products which have abrittle point considerably above room temperature. Therefore, whenpolyesters for the purposes of the present invention are made fromphthalic acid, the phthalic acid in general should be dilutedconsiderably with some other dicarboxylic acid, such as a polymethylenedicarboxylic acid, which will lower the brittle point.

In general, the brittle point occurs at higher temperatures as theconcentration of groups having polar activity increases in thepolyester. Thus, among the polymethylene glycols and dicarboxylic acids,those having the longer polymethylene chains tend to form the polyestershaving the lower brittle points. Polyesters containing no aromatic ringsand fewer than one ester group per 5 atoms in the linear chains willordinarily produce cured polymers which have brittle points below 40 C.

Resistance to liquid hydrocarbons, on the other hand, is greatest inthose rubbers prepared from polyesters having the largest number ofester groups in the linear chains.

In general, any linear saturated polyester is suitable for the purposesof the present invention if it possesses the requisite degree of lineargrowth and non-crystallinity as outlined above, if it contains on theaverage one or more ester groups per 20 total atoms in the linear.chain, or preferably at least one ester group per 12 atoms in thelinear chain, or still more desirably at least one ester group per 7atoms in the linear chain and if it is formed of divalent organicradicals, joined by ester groups, which contain no groups which wouldinterfere with cure. The most stable poly- 9 phur linkages, acetallinkages, ketone groups and various other structures either in thelinear chain or in substituted side chains. Most commonly,

however, these radicals will be made up of divalent aliphatichydrocarbon residues, particularly polymethylene groups or alkylsubstituted polymethylene groups. I

The same is true of the unsaturated polyesters except that unsaturationis present in certain of the divalent organic radicals and somecross-linking at the double bonds inevitably has occurred.

The polyester gums prepared as described above are most readily cured byintimately mixing them with a small amount of ,benzoyl peroxide andheating them to temperature above 105 C., and preferably to atemperature of about 125 C. The cross-linking activity of benzoylperoxide appears to be due to the fact that upon decompositionitgenerates free radicals which, because of their hydrogen deficiency,remove the active hydrogen atoms on the carbon atoms adjacent to theester groups of the polyesters, thus causing cross-linking between thesecarbon atoms at the free valences. Although benzoyl peroxide has beenfound the most effective cross-linking agent, other acyl peroxides, suchas lauryl peroxide, have also been found particularly effective. ganicperoxides, such ascertain of the ether peroxides, ketone peroxides,olefin peroxides, terpene peroxides (particularly ascaridole), peracidsand hydrocarbon peroxides, are sufflcientlyefiective to render themusable for the purposes of the present invention.

Other free radical generating substances, such as phenyl azide orcertain other azides, when used in sufiicient quantity also exert auseful cross linking action.

'In each case the free radical generating subtance is intimately mixedwith the polyester, as by milling, and the mixture is then cured underpressure in a mold at a curing temperature. The curing agent may beincorporated as a finely divided solid or in the form of a solution.When benzoyl peroxide is used, it may be in the form of finely dividedcalcium sulphate having precipit'ated thereon about 20 per cent ofbenzoyl peroxide.

The amount of curing agent required to give maximum tensilestrength'varies with the composition of the polyester, being affectedboth by the nature of the saturated components and by the amount ofunsaturation. When a given polyester gum is cured with increasingamounts of benzoyl peroxide, the tensile strength rises very sharply,passes through a peak and decreases slowly. With saturated polyestersthis peak is quite broad, the optimum amounts being from 3 per cent to 6per cent for sebacates and from per cent to 8 per cent for succinates.amount of unsaturation increases, the peak becomes much sharper and theactual amount of peroxide required for cure becomes much less. Thus fora sebacate polyester in which 4 moi per cent of the sebacic acid isreplaced by maleic acid, the optimum amount of peroxide is from 0.5 percent to 1 per cent; for a corresponding succinatemaleate, the optimumamount is from 1 per cent to 2 per cent. As the amount of maleic acidapproaches about 10 mol per cent, the amount of peroxide requiredbecomes still less and the peak becomes very critical. I

The polyester, prior to curing, will ordinarily be mixed with areinforcing pigment. The nature of the pigment employed has an importanteffect upon the tensile strength of the product.

As the 10 Two pigments have been found outstanding in producing curedproducts of high tensile strength.

- fine precipitated calcium carbonate having an Certain other oraverageparticle size of the order of 30 millimicrons, sold under the trade nameof Kalvan." The reinforcing carbon blacks are not desirable as pigmentssince they impede the curing action, making it necessary to employ largeamounts of benzoyl peroxide or excessively high curing temperatures, inorder to secure effective curing.

The optimum tensile strengths are obtained with Mapico 297 when thepigment is, present in amounts between about per cent and 150 per centby weight of the polyester. The optimum percentage of Kalvan" is in thevicinity of about 50 per cent by weight.

Other finely divided mineral fillers, of neither strongly acid norstrongly alkaline nature, such as aluminum oxide, talc, antimony oxide,titanium oxide or the various clays, may' be employed either alone or inmixture with Kalvan" or "Mapico 297.

The following specific examples will illustrate manners in which thepresent invention may be practiced: i

tilled sebacic acid, 44.5 mol per cent isopropylene glycol and 11 molper cent ethylene glycol (25 per cent excess glycols) together with asmall amount of zinc chloride as a catalyst wereplaced in a closed glassreaction vessel maintained at 200 C. and a slow stream or dry,oxygen-free hydrogen was bubbled continuously through the moltenmixture. A packed reflux column maintained at C. was attached to thereaction vessel. After about five hours no more water was evolved,indicating that substantially complete esterification had occurred. Thereflux column was then removed and the pressure in the system wasreduced to about six millimeters of mercury, the temperature beingmaintained at about 200 C. and the bubbling of hydrogen being continued.Glycol distilled over rapidly and after about fifteen minutes anincrease in the viscosity of the product was apparent. At the end ofabout eight hours the product was removed and found to be an exceedinglyviscous transparent liquid which crystallized slowly at room temperatureto a tough, flexible, rubbery, translucent solid. A portion of thissolid substance was quickly reduced to its viscous liquid statebymilling on cold rolls. About 3 per cent of finely dividedbenzoylperoxide was then thoroughly milled into the gum. Seventy-fiveper cent by weight of Kalvan" was then added to the gum on the rolls andthe milling was continued until the pigment was well dispersed. Thismixture was then cured under pressure for ten-minutes in a mold'heatedto C. The resulting product had a tensile strength of 3200 pounds persquare inch at an elongation of 650 per cent. The brittle point of theproduct was below 50 C. 1

Example 2 A polyester was prepared as described in Example 1 except thatmaleic acid replaced sebaclc acid to the extent of 4 mol per cent of thesebacic acid which was present. The resulting polyester was milled with0.75 per cent benzoyl peroxide and 150 per cent by weight of Mapico 297"and was cured at 125 C. The cured product had a tensile strength of 2600pounds per square inch and a brittle point in the vicinity of 50 C.-

Example 3 A polyester was prepared as in Example 1 except that thedicarboxylic acid mixture was made up of 97 mol per cent of succinicacid and 3 mol per cent maleic acid and the glycol mixture was made upof 50 mol per cent isopropylene glycol and 50 mol per cent ethyleneglycol. 25 per cent excess glycol being used. The polyester waspigmented with 150 per cent by weight of Mapico 297 and cured with 2 percent benzoyl peroxide. The cured product had a tensile strength of about2700 pounds per square inch at an elongation of 520 per cent.

Emample 4 Example 5 A polyester was prepared as described in Example 1using a dicarboxylic acid mixture containing 30 mol per cent phthalicacid, 67 mol per cent sebacic acid and 3 mol per cent maleic acid and a25 per cent excess of a glycol mixture containing 50 mol per centethylene glycol and 50 mol per cent isopropylene glycol. The resultingpolyester, pigmented with 100 per cent by weight of Mapico 297 and 1 percent benzoyl peroxide,

possessed a good tensile strength and reversible elasticity.

Example 6 0.95 mol of succinic acid, 0.05 mol of maleic acid and 1.05mols of trimethylene glycol were reacted in an enclosed glass vessel ata temperature of about 200 C., while bubbling continuously a stream ofdry, oxygen-free hydrogen through the reaction mixture for about 9hours. After a large portion of the water of reaction together with aportion of the free glycol had been removed from the reaction mixture bythis procedure, the substances remaining in the reaction mixture were ofsuiiiciently high molecular weight to permit subsequent heating in avacuum without excessive volatilization. The vessel and its contentswere then subjected to evacuation while allowing sufficient hydrogen toenter the system through a needle valve to agitate the reactant whilestill maintaining an absolute pressure of about 0.5 centimeter ofmercury. The reaction was allowed to proceed under reduced pressureuntil a degree of esterification in excess of 98 per cent oftheoretically complete esterification was reached. This degree ofesterification was reached by continuous heating for eight or morehours. After the desired degree of esteriflcation had been obtained,atmospheric pressure was reestablished by the admission of hydrogen, and0.05 per cent by weight of betanapthol was added to retard subsequentoxidation of the reaction mixture upon exposure to air. The moltenreaction product was then poured Example 7 A reaction similar to thatdescribed in Example 6 was carried out using 0.5 mol of sebacic acid,0.2 mol of succinic acid, 0.2 mol of adipic acid, 0.1 mol of fumaricacid and 1.05 mols of diethylene glycol. A similar rubber-like productwas obtained upon curing,

Example 8 A reaction similar to that described in Example 6 was carriedout using 0.95 mol of succinic acid, 0.05 mol of maleic acid and 1.05mols of diethylene glycol. This product when cured showed inappreciableswelling in toluene after three hours immersion, whereas soft vulcanizednatural rubber in this time swells several hundred per cent. The brittlepoint of the product was -35 C.

Example 9 One hundred grams of the polyester produced in Example 3 weremixed on the rolls with 2 grams lauryl peroxide and then cured in a moldfor five minutes at C. The product was a transparent, flexible rubberysheet.

Example 10 One hundred grams of the polyester produced in Example 3 weremixed on the rolls with 50 grams of Micronex carbon black and 6 gramstertiary butyl hydroperoxide and cured in a mold for one hour at 170 C.The product was a very tough sheet of high modulus having a tensilestrength of about 1800 pounds per square inch.

Example 11 Four grams of the polyester produced in Example .3 wereintimately mixed with 0.2 gram of phenyl azide and heated for fourminutes at about C. A rubber-like mass was obtained having goodreversible elasticity.

The properties of the cured polyesters of the present invention may bemodified by theaddition of any of the compatible rubber compoundingingredients, such as softeners, plasticizers, pigments, bitumens,paraffin waxes and similar materials. The uncured polyesters,particularly those containing olefinic unsaturation, may be mixed withvarious cellulose derivatives, such as cellulose acetate or celluloseacetate butyrate, in any amounts and then may be subsequently cured.Although these substances are not always permanently compatible beforecuring, they remain permanently interdispersed after curing. When thecellulose derivatives are present in substantial amounts, the productsare not rubberlike, the polyesters merely actingas plasticizers andtougheners for the cellulose derivatives. In a similar manner theuncured polyesters, particularly those containing olefinic unsaturation,may be mixed in varying proportion with other synthetic resins, such asthe methacrylate resins, polyvinyl acetate, polyvinyl acetate-vinylchloride, or polystyrene and may then be cured,

The rubber-like substances of the present invention have a very wideapplication. They may be used in sheet form for gaskets, printersblankets, or similar articles or they may be molded as thermosettingmolding materials. Alternatively, uncured polyesters may be dissolved insuitable solvents together with vulcanizing agents and the resultingsolution may be employed to impregnate or coat cloth or other materialsfor forming oilcloth, raincoats, or similar articles. The uncuredpolyesters may also be employed as thermosetting adhesives when mixedwith benzoyl peroxide. These materials may also be used as insulation onelectrical conductors in any manner in which rubber has been employed.For this purpose the uncured polyester, mixed with curing agents andsuitable fillers and'modifying ingredients may be extruded continuouslyon wire and continuously cured. Other uses will be obvious from thedescription above.

Although the invention has been described in terms of its specificembodiments, certain modifications and equivalents will be apparent tothose skilled in the art and are intended to be included within thescope of the invention, which is to be limited only by the reasonablescope of the appended claims.

What is claimed is:

1. A cured synthetic rubber comprising a product obtained by heating toa temperature above about 105 0., with a small amount of benzoylperoxide, a fusible ethylene glycol-isopropylene glycol-sebacicacid-maleic acid polyester the ingredients of said polyester enteringinto the polyester in the proportions of about mol per cent of ethyleneglycol, about 80 mol per cent of isopropylene glycol, about 97 -mol percent of sebacic acid and about 3 mol per cent of maleic acid, the esterchains of said polyester having two ends, the number of ester groups insaid polyester constituting at least 98 per cent of the total number ofester, hydroxyl and carboxyl groups.

2. A cured synthetic rubber comprising a product obtained by heating toa temperature above about 105 C., with a small amount of benzoyl whereinthe isopropylene residue constitutes mol per cent of the totalisopropylene and ethylene residues and the sebacate residue constitutes97 per cent of the total sebacate and maleatc residues, and wherein thenumber of ester groups in the average molecule of said polyesterconstitutes at least 98 per cent of the total number of ester, hydroxyland carboxyl groups in said polyester.

CALVIN S. FULLER.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,071,250 Carothers Feb. 16, 19372,252,271 Mathis Aug. 12, 1941 2,255,313 Ellis Sept. 9, 1941 2,308,494D'Alelio Jan. 19, 1943 2,388,319 Fuller Nov. 6, 1945 FOREIGN PATENTSNumber Country Date 500,547 Great Britain Feb. 8, 1939 OTHER REFERENCESVincent, Ind. and Eng. Chem., vol. 29, pages 1267-1269, Nov. 1937.

